Structure for joining by fusion-welding of ferritic steel with austenitic steel



Sept. 4, 1962 F. ZIMMER 3,052,016

STRUCTURE FOR JOINING BY FUSION-WELDING OF FERRITIC STEEL WITH AUSTENITIC STEEL Filed March 19, 1958 2 Sheets-Sheet 2 14 00 12 ..12 a n 10 10. w I 8 8 I o 5 9 6 x Cr 10 N Cr Mo C o 1 o INVENTOR FRA/VT/Jfk Z/MM eve nited States This invention relates to the joining by fusion-welding of ferritic steel with austenitic steel, such steels being more particularly used in the steam generating stations which employ steam superheated at a temperature higher than about 550 C.

Owing to the very different physical and metallurgical features of the two types of steel above mentioned, the joint is the seat of harmful phenomena, of which the following are mentioned:

Owing to the different coefficients of expansion of the two types of steel (14.10 for the ferritic steel and 18.10 for the austenitic steel), very high stresses are produced in the joint during temperature variations. Such coeflicients of expansion are mean coefficients be tween 20 and 600 C. on the basis of cm./cm. per degree centigrade.

The stresses produce at high temperature, plastic deformations due to creep in the ferritic steel (which has half the resistance, when hot, of the austenitic steel).

During the cooling, the said deformations produce stresses in an opposite direction, which may exceed the elastic limit of the metal in a cold state, thus producing new plastic deformations in an opposite direction. Such repeated stresses and plastic deformations produce a strong fatigue of the ferritic steel near to the surface of the fusion-welding; cracks are thus produced in the ferritic steel which endanger the resistance of the joint.

Another phenomenon which has a harmful influence upon the resistance of the joint is the migration of the carbon from the ferritic steel into the austenitic steel, accompanied by an increase of the size of the grain in the decarburized zone.

The characteristic features of the latter zone are thus affected, namely its resistance in the cold state and in the hot state is appreciably reduced, the metal becomes brittle and very sensitive to corrosion-fatigue.

The migration of the carbon is mainly due to the high percentage of chromium in the austenitic steel, such chromium having a strong afiinity for the carbon.

Still another harmful phenomenon is the corrosionfatigue which appears mainly in the decarburized zone of the ferritic steel near the surface of the fusion-welding. Cracks are thus produced which start at the surface and are prolonged along the line of fusion where the stresses due to the different coefiicients of expansion are the strongest.

Various modes of producing joints are known, which reduce by different amounts, the harmful phenomena above described.

Among the joints which reduce the stresses due to the different coefficients of expansion, a joint has been described which comprises three intermediary bufier pieces, the coefiicients of expansion of which are respectively 15.10- l6.l() and 17.10 The weldings between the intermediary buffer pieces have corresponding coefiici-ents of expansion. Another similar joint comprises a fusion-welding consisting of 5 buffer-layers deposited by 5 electrodes, the coefficients of expansion of which are: 1410- 1510- 1610*, 17.10 and 1810*.

The said joints reduce to 25% the stresses due to the different coefficients of expansion when compared with an ordinary direct joint between ferritic steel and austenitic steel.

has

However, they have the following drawbacks:

The first mentioned mode of joining requires four weldings with four different electrodes; this makes the process complicated and unpractical.

The second mentioned mode of joining comprising 5 layers deposited with 5 different electrodes, is accompanied by considerable technical difficulties.

The decarburizing of the ferritic steel is prevented by the insertion between the ferritic steel and the austenitic steel, of a welding metal or of an intermediary bufferpiece of stabilized ferritic steel. Such a method has the drawback that it does not allow of simultaneously reducing the stresses due to the different coefiicients of expansion.

It is known that the corrosion-fatigue may be reduced by a protective layer deposited by welding upon the finished joint, covering the line of separation between the ferritic steel and the austenitic steel.

The metal forming the said protective layer has generally a coefficient of expansion which is intermediary between that of the ferritic steel and that of the austenitic steel, and has a great resistance of oxidation.

Such a method does not reduce the stresses due to different coefiicients of expansion, and does not prevent the decarburizing of the ferritic steel.

A joint made according to the present invention allows of suppressing all the harmful phenomena above mentioned, that is: the stresses due to the different coeificients of expansion, the decarburizing of the ferritic steel, and the corrosion-fatigue in the decarbun'zed zone.

Such results are obtained according to the invention by the use of a transition piece of continuously variable composition, the coefficient of expansion of which increases in a continuously progressive manner from F 14.10- (coefficient of ferritic steel), at one end, to

18.10 (coefiicient of austenitic steel) at the other end of the transition piece. The transition piece may be made by a method of powder metallurgy.

The basic principle and the advantages of the transition piece according to the invention will appear from the description which follows, with reference to the accomparliying drawings which are given by Way of example on y.

Referring to the drawings,

FIG. 1 (centre) is longitudinal section through the transition piece witth the following diagrams:

(a) a diagram of the variation of the coeflicient of gii gmiion in the transition piece, at the upper part of (b) a diagram of the variation of the composition of the transition piece at the lower part of FIG. 1.

FIG. 2 (centre) shows a mixture made of a powder A having a coeflicient of expansion equal to l4.10 with a powder B having a coefficient of expansion equal to 15.10 The upper diagram of FIG. 2 shows the variation of coefficient of expansion. The lower diagram of FIG. 2 shows the variation of composition.

FIGS. 3 and 4 show modifications of the invention.

Referring to H6. 1, the transition piece 1 has a conical shape with a cylindrical bore, and is ended at both ends with bevelled edges for the fusion-welding. The transition piece may however have another shape, cylindrical for instance.

The conical shape of the piece, the thickness of which diminishes from one end to the other, takes into account the respective resistances in a hot state of the ferritic and austenitic steels, the latter being about twice as resistant as the former. The transition piece is therefore a piece of uniform strength. The diagram shown below the piece 1 in FIGURE 1 shows the variation of the composition of the austenitic alloy of the joint along its length.

At the left-hand end of the piece 1, that is on the side of the welding with the ferritic steel, the percentages of special elements of the alloy are the following: nickel 50%, chromium cobalt 10%.

Such a composition, which is also that of the welding with the ferritic steel, has the following objects: to ensure a coefficient of expansion at one end of the transition piece, which shall be equal to that of the ferritic steel (14.10 thus avoiding the dangerous stresses due to different coefficients of expansion. To avoid all danger of decarburizing of the ferritic steel in the weld and in the transition piece by incorporating in the welding metal and in the metal of the transition piece a high percentage of nickel (minimum 50%), which element has a very small affinity for the carbon and prevents thus a migration of the element. To give to the weld and to the metal at that point, the required strong mechanical and chemical resistances when in a hot state.

By avoiding the stresses due to the different coefficients of expansion and also the decarburizing of the ferritic steel, a corrosion-fatigue of the ferritic steel is also avoided.

The composition of the metal of piece 1 at the righthand end, that is on the side of the austenitic steel, is similar to that of the austenitic steel 18/8 (chromium 18%nickel 8%) and of the welding metal which is used on that side.

In addition to the special elements shown in the diagram (lower part of FIG. 1), the alloy comprises in all its parts a percentage of from 0.05% to 0.1% of carbon and of from 0.5% to 1% of niobium, the latter being the stabilizing element.

Special elements such as molybdenum, vanadium, tungsten, may be added to the alloy in order to increase its resistance at high temperature.

The powder metallurgy allows of effecting for the same purpose non-metal additions, such as oxides, for instance thoria (oxide of thorium).

The composition of the metal in the various sections of the transition piece may therefore be different from that shown in the diagram of FIGURE 1, at its lower part.

However, the special additions are made in such a manner that the coefficients of expansion in the various sections of the transition piece shall change progressively without any discontinuity, according to the upper diagram of FIGURE 1.

Under these conditions, the difference between the coefiicients of expansion of two neighboring sections becomes so small that the resulting stresses are practically suppressed, which is an important advantage of the present invention.

The manufacture of the transition piece by means of the powder metallurgy includes a pressing into compacts of the metal powder in a mould having the required shape, followed by a sinteriug at high temperature of the compacts. For the manufacture of the alloyed pieces, the metals which enter their composition may be introduced by mixing the separate powders with one another, or use maybe made of an alloy reduced to powder form.

Either method may be used for the manufacture of the transition piece of variable composition according to the invention.

According to the first method, the powders of elements forming part of the com-position are introduced into a vertical mould, after they have been carefully mixed in proportions which vary along the various sections, as shown in the lower diagram of FIGURE 1, or in a similar manner. The introduction of separate powders of metals such as iron, nickel, cobalt, offers no difliculties. On the other hand, metals such as chromium and aluminium which form a film of refractory oxide, preventing the diffusion of the metals during the sintering process, may be introduced into the mixture in the form of a powder of a binary alloy (for instance an intermetallic compound Co Cr containing 57% chromium and 43% cobalt. As to the carbon, it may be introduced in the form of graphite powder or of a carbide, for instance carbide of tungsten.

The mixed powders are then pressed into compacts at a pressure comprised between 40 and kgs./mm. according to the composition. The compacts are sintered in a protective atmosphere, in a suitable furnace known per se. In order to reach a density approaching that of the massive metal (that is, without porosity), for obtaining the required mechanical properties, the sinteriug may 'be replaced by a hot pressing with application of heat, or by a sinteriug made in several stages with an intermediary pressing.

It is also possible to subject the piece after sintering to a forging or to a drawing, at a high temperature, which process is followed by a thermal treatment for obtaining the optimum mechanical characteristics.

The second method above referred to, employing alloys of different compositions reduced to powder form, has the advantage of suppressing the difiiculties of introduction of easily oxidizable elements, such as chromium and aluminium.

The variable composition of the transition piece is obtained by such a method in the following manner:

Powders are prepared of a certain number of alloys having coefiicients of expansion which are sufficiently near to one another, for instance 1410-, 15.10- 16.10- 17.10 18.10* and having the corresponding compositions according to the lower diagram of FIGURE 1.

The powders of two neighbouring compositions which are introduced into the mould, are intimately mixed in proportions such that the mixture passes progressively from one composition to the other. FIG. 2 shows a mixture of a powder A having a coefficient of expansion of 14.10- with a powder B having a coefiicient of expansion of 1510- The composition of the powder A is: 50% Ni, 10 Cr and 10 Co. The composition of the powder B is: 40% Ni, 12% Cr and 4% Co.

By mixing the powders according to the diagram 2 at the lower part of FIG. 2, the composition of the metal between the sections A-A and BB (FIGS. 1 and 2) changes after sintering, progressively in a continuous manner, and the coefiicient of expansion changes also progressively in a continuous manner as shown by the diagram at the upper part of FIGURE 1.

The method is the same for the remaining parts of the transition piece.

The sintering of the compressed material and the other operations are the same as in the first method.

The transition piece made by the powder metallurgy has the following additional advantages:

Reduced tolerances of the dimensions; No machining;

No loss of material;

Economical cost of production.

The said method is particularly applicable to mass production, for instance of transition pieces for superheaters working at a high temperature, which is a particularly interesting application of the present invention.

It is possible to standardize the production of the transition pieces, thus further reducing their cost of manufacture.

According to a modification shown in FIG. 3, the transition piece consists of a transition ring 2 of variable composition made by the metallurgy of powders, inserted by flash-welding between a short tube of ferritic steel 3 and a short tube of austenitic steel 4.

In the said FIGURE 3, the said transition piece is accompanied, by diagrams showing respectively:

Above, the variation of the coefiicient of expansion; below, the variation of the composition of the metal.

In certain cases, it may be sufficient to use a transition piece consisting only of a ring 2 and piece 1 of ferritic steel connected by flash-welding.

The main additional advantages of such a mode of carrying the invention into efiect are:

(a) A flash-welding at the ends may be automatically regulated, which allows of obtaining strong weldings and of avoiding the human factor.

(b) The said transition piece allows of carrying out a joining of ferritic steel with austenitic steel, by using conventional ferritic and austenitic) electrodes, without any special precautions being necessary.

FIG. 4 shows another modification according to which a transition piece or a piece of any shape made by the metallurgy of powders, comprises three parts:

A ferritic part 3, a transition part of variable composition 2, and an austenitic part 4, the lengths, section and composition are in each case adapted to the desired result above mentioned.

For instance a turbine blade made of stainless steel intended to be welded upon the rotor made of ferritic steel, is made by the metallurgy of powders and comprises the following parts: the blade itself of stainless steel, foot of blade consisting of the portion of variable composition, and an end territic portion. The three parts are integral with each other.

The second modification has upon the first one the additional advantage of suppressing the two flash-weldings.

The three processes above described may be combined to form transition pieces or any other pieces.

What I claim is:

1. A structure comprising members of territic and anstenitic steels and a transition piece for insertion between the said members which transition piece is an alloy which varies continuously and progressively so that the nickel varies from 50% at the first end to 8% at the other end, chromium varies from 10% at the first end to at the other and cobalt varies from 10% at the first end to 0% at a point substantially in the middle of the piece, with the balance throughout the piece being essentially iron, and the said transition piece is welded at its first end to the ferritic steel member by a weld having the composition of said first end and is welded at its other end to the austenitic steel member by a weld having the composition of said other end.

2. A structure as claimed in claim 1, in which the transition piece is of tubular form.

3. A structure comprising members of ferritic and austenitic steels, and a transition piece for insertion between the said members which transition piece is an alloy and comprises three parts, namely a central part, a ierritic part and an austenitic part, the central part having a composition which varies continuously and progressively so that nickel varies from at the first end to 8% at the other end, chromium varies from 10% at the first end to 20% at the other end, cobalt varies from 10% at the first end to 0% at a point substantially in the middle of the central part, with the balance throughout the central part essentially iron, which central part is bonded at its first end with the ferritic part and is bonded at its other end to the austenitic part and the three part piece is bonded by its ferritic steel part to the ferritic steel member by a weld of the same composition as that of the territic steel part and by its austenitic part to the austenitic steel member by a weld of the same composition as that of the austenitic part.

References fitted in the file of this patent UNITED STATES PATENTS 1,966,403 Durham July 10, 1934 2,067,342 Rauwenhofi Ian. 12, 1937 2,100,880 Stansel Nov. 30, 1937 2,297,554- Hardy et a1 Sept. 29, 1942 2,380,071 Planett July 10, 1945 2,431,660 Gaudenzi Nov. 25, 1947 2,763,923 Webb Sept. 25, 1956 2,769,227 Sykes Nov. 6, 1956 

